This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Soluble guanylate cyclase (sGC), a 150 kD heterodimeric protein, is the primary nitric oxide (NO) receptor in the cell. NO binds to heme in the N-terminal domain of the beta subunit, stimulates the production of cGMP and leads to NO-dependent signaling cascades. In vitro studies have shown that a probably allosteric conformational change takes place when ligands bind. In addition to the physiological ligand NO, there are effector ligands, typified by YC-1 that act in concert with carbon monoxide (CO). We request beam time to perform small angle x-ray scattering studies on soluble guanylate cyclase in order to characterize ligand binding associated conformational changes. Several different domains and constructs of both human and Manduca sexta sGC have been expressed by recombinant methods and are available, or will become available in the near future. Preliminary analytical ultracentrifugation studies demonstrate that the conformational change is large enough to be characterized by SAXS and that the protein does not aggregate significantly under the conditions of the centrifugation experiment.